In swash plate type or bent axis type hydraulic pumps or hydraulic motors and radial piston type hydraulic pumps or hydraulic motors, a piston is slidably fitted in each one of cylinder bores on a cylinder block which is coupled with a rotational shaft. For example, in the case of a swash plate type hydraulic pump, a cylinder block is accommodated within a casing, and a swash plate is disposed face to face with the cylinder block. A rotational shaft is extended through the swash plate and coupled with the cylinder block for rotation therewith. A plural number of cylinder bores (normally an odd number of cylinder bores, e.g., 5 or 7 cylinder bores) are formed in the cylinder block at intervals in the rotational direction. Shoes of the same number as the cylinder bores are connected with the respective pistons and held in sliding engagement with the swash plate.
Upon driving the rotational shaft, the cylinder block is put in rotation along with the rotational shaft, putting the pistons in the respective cylinder bores in reciprocating movements in a certain stroke range depending upon the angle of the swash plate. By rotation of the cylinder block, each one of the cylinder bores is connected either to an intake port or an output port in a switching fashion. Namely, as a piston is projected out of a cylinder bore, which cylinder bore is connected to an intake port to suck in operating oil. On the other hand, as a piston is retracted inward of a cylinder bore, which cylinder bore is connected to a discharge port to deliver pressurized oil.
In a swash plate type hydraulic pump as described above, it is necessary for a smooth operation of the pump to improve the quality of sliding performances of sliding surfaces on the respective pistons and cylinder bores. In order to lessen sliding friction between sliding surfaces, it has been proposed to form a sliding layer of a low friction copper alloy on sliding surfaces of cylinder bores in Patent Literature 1 below. In the case of Patent Literature 1, a sliding layer is formed by depositing a low friction copper alloy on inner sliding surfaces of a cylinder bore, followed by sintering to minimize sliding friction.
However, in order to form a low friction copper alloy layer on inner surfaces of cylinder bores as mentioned above, it becomes necessary to resort to a complicate deposition process which would result in an undesirable increase in production cost. In this regard, taking into consideration that a casing of a hydraulic rotary machine is entirely filled with operating oil when in operation, it would be more advantageous to utilize the operating oil as a lubricant in improving the sliding friction between a piston and a cylinder bore rather than resorting to a complicate low friction layer deposition process. For this purpose, it is a paramount requisite to form and constantly maintain a filmy layer of operating oil on inner sliding surfaces of each cylinder bore which are in sliding contact with a piston. Namely, inner surfaces of a cylinder bore should have a high oil retaining capacity.
In this regard, Patent Literature 2 below teaches an example of imparting oil retaining properties to a sliding surface. Namely, Patent Literature 2 teaches to improve oil retaining properties of plane sliding surfaces of engine shift forks by forming thereon a multitude of micro dimples. Further, Patent Literature teaches that micro dimples of this sort can be formed by a cutting, grinding or plastic texturing operation or otherwise by shot peening.    Patent Literature 1: Japanese Laid-Open Patent Appln. H7-167041    Patent Literature 2: Japanese Laid-Open Patent Appln. 2001-280494